1. Field of the Invention
The present invention relates to a piston-cylinder assembly, comprising a cylinder, a piston rod with a piston guided axially by a piston rod guide in the cylinder, and a distance detection system for detecting the position of the piston in the cylinder.
2. Description of the Related Art
Devices for determining the distance traveled by a piston of a hydraulic, pneumatic, or hydropneumatic system are known, for example, from U.S. Pat. No. 4,577,509 (Moser), in which the cylinder includes an additional tube in the interior cylindrical space thereof which forms a variable capacitor with the piston rod. The sensor and the measurement value transmitter are designed to extend down the entire length of the cylinder or of the piston rod and therefore require a corresponding amount of space.
Piston-cylinder assemblies with motion detection devices are also known from U.S. Pat. No. 6,318,524 (Lutz et al.). These assemblies comprise a cylinder and a piston rod with a piston is guided axially in a piston rod guide. The motion detection device comprises a transmitter and a receiver, where the movement of the piston is determined from the transit time of the waves generated by the transmitter. The transmitter and the receiver are installed inside the piston rod guide. The waves reflected from the piston-piston rod assembly are evaluated to determine the movement of this assembly.
Devices for determining the distance traveled by a piston in a cylinder for hydraulic or hydropneumatic systems such as vibration dampers are also known, for example, from German reference DE 36 43 290 C2. Here the piston on its piston rod is mounted with freedom to slide axially inside at least one damping medium-filled cylinder. A Hall sensor connected to the piston rod is used to detect the level. A part of the natural outside surface of the cylinder and at least a part of the natural surface of the piston rod are used as the measurement section.
An object of the present invention is to create a standard distance detection system that is compact, can be built at low cost, and which can be integrated into any type of shock absorber, piston-cylinder assembly, or automatic level-control element.
The object of the present invention is met by a distance detection system comprising a sensor and a transmitter, wherein the sensor is mounted on either the piston or the bottom of the cylinder and wherein the transmitter travels a distance different from that traveled by the piston upon axial displacement of the piston. The difference between the two distances traveled by the piston and the transmitter, respectively, being used to determine the position of the piston.
The distance detection system of the present invention may be integrated into shock absorbers, piston-cylinder assemblies, or automatic level-control elements, even if the assemblies have different lengths. The need to keep large numbers of different types of sensors and transmitters in stock for different lengths is obviated. Instead, systems of standard parts can be used.
The transmitter may be supported by at least one spring on the piston and by at least one spring on the bottom of the cylinder, wherein these springs have different spring rates. Furthermore, several springs may be connected in series, where at least one spring is supported in the piston area, while the other springs are supported on the bottom area of the vibration damper and thus hold the transmitter at a certain position relative to the sensor, the exact position being a function of the position of the piston.
As stated above, the sensor may be mounted on either the piston or the bottom of the cylinder. In one embodiment of the present invention, the spring or springs between the part that the sensor is mounted and the transmitter are designed with a higher spring rate than the other spring or springs.
To ensure the satisfactory guidance of the springs when several springs are connected in series in especially long assemblies, a guide element may be provided for guidance on the inside wall of the cylinder between each pair of adjacent springs.
The transmitter may be guided so that it slides axially on the sensor or so that the transmitter is coaxial to and a certain distance away from the sensor.
The distance detection system may also be designed so that it is based on the principle of a gap measurement. That is, the end surface of the sensor may cooperate with an end surface of the transmitter.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.